Your display name is what friends see, and you can change it whenever. Your username is your @handle, and you can change that too.
How to change it
Open your profile editor
From your profile, tap Edit profile.
Edit the username field
Allowed: 3-30 lowercase letters, numbers, and underscores. Usernames are unique, if it's taken you'll see an error. Names that could pass for PopIn staff (anything like "admin", "support", "official" or "popin", including look-alike spellings) are reserved, and the same goes for display names.
Tap away from the field
Profile edits save on their own, no Save button. Watch for the little Saved tick, then your new handle is live immediately.
Once you change your handle, the old /profile/oldhandle link no longer points to you, and your old username becomes available for someone else to take. If you've shared your profile link anywhere lasting (a bio, a business card), update it after you switch.
Pick a handle you'll be happy with for a while. Changing it is easy, but anyone relying on your old one will need the new one.